The simple answer is that each REST call made to the Azure Storage Service is counted as a single transaction. This means that each time you query your table or check the size of a queue or upload a blob you will call the Azure Storage REST api and it will be classed as a transaction. It also means that if you are doing a full table query and you start to get continuation tokens you will get multiple transactions.
For a fuller description see the following blog post:
http://blogs.msdn.com/b/windowsazurestorage/archive/2010/07/09/understanding-windows-azure-storage-billing-bandwidth-transactions-and-capacity.aspx